1. Field of Invention
This invention relates to coating compositions. More particularly, this invention relates to emulsion or dispersion coatings of carboxyl functional polymers.
2. Prior Art
The use of carbodiimides alone to crosslink emulsion-based coatings is known as is the use of metal driers alone to crosslink emulsion coatings. However, in neither case is a satisfactory degree of crosslinking achieved because the films crosslinked in this fashion exhibit reduced properties when compared to films and coatings crosslinked employing different reaction mechanisms.
The applicants, on the other hand, have overcome these problems by utilizing a carboxylic acid containing coating composition in admixture with a carbodiimide and a certain metal catalyst. By employing this critical mixture of components, the degree of crosslinking and the resulting physical and chemical resistance properties of cured coatings are significantly increased over the properties of coatings cured with a carbodiimide or a metal alone. In addition, in some applications, the cure time for the coating is significantly reduced as compared to the cure time when a carbodiimide or a metal alone is used.
While films having good chemical and physical resistance properties have been obtained utilizing melamine/formaldehyde or urea/formaldehyde curing agents, these formaldehyde-based materials are highly undesirable given current concerns about formaldehyde. In addition, although films have been obtained utilizing low or medium solids, two package polyurethane systems, these acrylic or polyester based polyurethanes are also highly undesirable because of the organic solvents which are emmitted during spray application of the coating and the toxicity of certain polyisocyanate reactants. By employing the combination of a carbodiimide and a metal as the crosslinking system, the undesirable properties of polyisocyanates and formaldehyde-containing curing agents are eliminated.
In U.S. Pat. No. 4,321,394, the reaction of carbodiimides and hydroxyl group containing compounds in the presence of a tin catalyst is disclosed. The patent, however, does not disclose the use of a carboxylic acid containing polymer emulsion or the use of a metal as a crosslinking catalyst.
U.S. Pat. No. 4,487,964 discloses a method for preparing mixed aliphatic/aromatic polycarbodiimides which may be employed as crosslinking agents for carboxylated latices. However, this patent does not disclose the use of a metal crosslinking catalyst of the type disclosed in the instant invention.
U.S. Pat. No. 4,612,054 discloses a coating composition containing a carbodiimide. The carbodiimide may be used in conjunction with conventional drying agents, such as metal salts, to crosslink alkyd or polyacrylate resins. However, there is no disclosure that a metal can be used to catalyze the crosslinking activity of the carbodiimide or that metal salts like transition metal ammonium carbonates or transition metal chlorides may be employed to achieve improved cured state film properties.
Accordingly, it is an object of this invention to prepare water-based coating compositions.
It is the further object of this invention to prepare carbodiimide and metal salt containing emulsion or disperson water-based coating compositions.
It is also an object of this invention to prepare carboxylic acid-containing, emulsion, dispersion, or soluble based coating compositions having a greater degree of crosslinking and thereby improved physical and chemical resistance when cured.
It is a further object of this invention to prepare a low temperature curing emulsion or dispersion coating.
It is yet another object of this invention to prepare highly crosslinked chemically and physically resistant coating compositions which do not contain formaldehyde-based curing agents.
It is another object of this invention to prepare acrylic emulsion or polyurethane dispersion based coatings which do not need to be reacted with polyisocyanates.
These and other objectives are obtained by preparing the coating compositions of the instant invention.